Swim: The desert sands don't blanket the entire earth. This will come in handy in those spots where you have to swim.
Treading water in the direction you desire. In Florida, you better be able to do this fast if you do it at all.
In Wasteland, the Swim skill can be increased by swimming on the surface map by jumping in the river near Sleeper Base, and drifting down past Las Vegas, Needles, Quartz, and on down past the Mine Shaft into the lake. The lake itself is too calm to raise your Swim skill. You can also raise Swim in the Las Vegas Sewer, around the island under the Temple of Blood in Needles, and the swimming pool in the Stage Coach Inn, in Quartz. The skill naturally maxes out at level 6 given enough practice.